The present invention relates generally to a source for generating atomic species and more particularly to a laser sustained discharge atomic beam nozzle source for generating an intense beam of atomic species having high kinetic energy.
Many types of gas discharges are used to excite materials for analysis via atomic emission spectrosocopy. These discharges are produced by electric fields with a range of frequencies: dc arcs (constant fields), ac arcs and sparks (1 kHz or less), inductively coupled plasmas (20-50 MHz), and microwave induced plasmas (about 2.5 GHz). All of these sources require some physical device to support the discharge: arcs and sparks require electrodes, the inductively coupled plasma uses an induction coil, and microwave plasmas employ a resonator or waveguide. Recently, free-standing continuous discharges have been produced by focusing the output of a sufficiently powerful cw-CO.sub.2 laser into inert gases, molecular gases and mixtures thereof at atmospheric pressures or above. The discharge resides near the focus of the laser beam independent of any physical support, and does not require a gas flow to stabilize the plasma as do some sources. Because the discharge is maintained by using optical frequencies (30 THz) the plasma is called a "continuous optical discharge" (COD). A review article entitled "Evaluation of the Continuous Optical Discharge for Spectrochemical Analysis," by David A. Cremers, Frederick L. Archuleta, and Ronald J. Martinez. Spectrochimica Acta 40 B, 665 (1985), reivews the characteristics of such discharges as well as the contributions to the scientific literature thereon. Although cw-laser radiation can maintain the continous optical discharge, the output power of such light sources is generally insufficient to initiate the discharge. Consequently, such plasmas can be initiated using conventional electrode sparks or by the spark produced by a focused laser pulse superimposed on the focal volume of the cw-laser beam used to maintain the plasma. The small spark plasma contains a high density of electrons which act as an absorbing center for the cw-laser beam. It is believed that at laser frequencies which are typically above the plasma frequency, absorption occurs mainly via free-free transitions associated with electron-ion collisions (inverse Bremsstrahlung). The temperature of the continuous optical discharge can approach that obtained by sparks (20,000 K. or higher) and is due to the penetration of high frequency optical radiation into the core of the plasma which is typically of the order of 1 mm in diameter. By comparison, at radio and microwave frequencies, which are below the plasma frequency, plasma heating occurs through direct plasma-electric field interactions characterized by much larger absorption coefficients. Consequently, only the outer layers of these plasmas are heated directly by the electric fields. The higher temperature of the continuous optical discharge is also related to its greater operating energy density compared to more conventional discharges.
The continuous optical discharge technique has been combined with a nozzle in "Nozzle Flow In A Laser-Heated Hydrogen Rocket," by Nelson H. Kemp and Robert G. Root, J. Spacecraft 16, 65 (1979). Therein the authors described the use of a continuous optical discharge to provide the energy source to heat a working fluid which then expands through a nozzle, thereby producing thrust in the usual manner for space propulsion. That the gas is heated in bulk by the continuous optical discharge can be seen by FIG. 2 thereof where it is shown that heating is significant approximately 3 cm from the laser throat, and from page 66, column 2, where significant convective losses are discussed. No attention is given to maximizing the output velocity of the atoms produced thereby but rather to maximizing the throughput as is evidenced by the huge laser powers under consideration and the large nozzle radii contemplated (10 kW to 5 MW and 0.93 to 20.8 mm, respectively).
With the advent of space shuttle flights bearing retrievable specimens, a serious chemical etching of the craft's surfaces has been detected along with a pronounced glow near the shuttle surfaces exposed normal to the direction of flight. The glow and etching have been correlated with O-atom density. The intensity of O-atoms in low earth orbit is about 10.sup.15 O-atoms/s-cm.sup.2. In order to simulate in-flight conditions in a ground-based facility, an intense source (&gt;10.sup.15 O-atoms/s-cm.sup.2) of O-atoms having a translational velocity of approximately 8 km/s (=5 eV), the velocity of spacecraft in low-earth orbit, is needed. Initial modeling of oxygen etching of space shuttle surfaces has shown that oxidation-resistant coatings need to be developed to increase the operational lifetime of critical components.
Data are needed to model the glow and etching phenomena with the goal of developing such long-lived materials useful for spacecraft in low-earth orbit. Pursuant to this goal, useful data to be obtained from a ground-based O-atom source would consist of angular and recoil-energy distributions for both reflected O-atoms and reaction products, velocities of incident O-atoms, and mass spectra to identify reaction products.
The production of intense high energy, low mass (&lt;40 amu) beams is technically difficult. The two principal techniques employed prior to the subject invention use dc arcs and charge exchange, but have a number of disadvantages. Current high intensity dc arc beam sources produce beam velocities of up to 4 km/s and require 8-12 kW of power input. The disadvantages of these devices are the large input energy and cooling requirements, instabilities in the arc due to electrode erosion, reduced O-atom velocities due to boundary layer cooling of the arc, and the high pumping speed requirements for the vacuum system due to the required high gas loads. Charge exchange sources, by contrast, suffer from intensity limitations at energies &lt;100 eV because of space charge defocusing. Although attempts to overcome this problem using electron neutralization of the beam have been attempted, such sources are used primarily for producing large beam fluxes at energies greater than 100 eV. Several radio-frequency discharge designs exist for O-atom beam sources, but these produce translational energies of less than 1 eV (1.6 km/s).
Accordingly, it is an object of the subject invention to provide an apparatus for generating a beam of atoms having high kinetic energy.
Another object of the subject invention is to provide an apparatus for generating intense beams of atoms having high kinetic energy.
Yet another object of our invention is to provide an apparatus for investigating the reaction of oxygen atoms having high kinetic energy with a target.
Additional objects, advantages and novel features of the invention will be set forth in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combination particularly pointed out in the appended claims.